1. Field of the Invention
The invention concerns an illumination system for wavelengths xe2x89xa6193 nm as well as a projection exposure apparatus with such an illumination system.
In order to be able to further reduce the structural widths of electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. Lithography with very deep UV radiation, so called VUV (Very deep UV) lithography or with soft x-ray radiation, so-called EUV (extreme UV) lithography, is conceivable at wavelengths smaller than 193 nm, for example.
2. Description of the Prior Art
An illumination system for a lithographic device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least 4 pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided, and the point plasma light source is imaged in an annular illuminated surface by means of a condenser, which has five aspherical mirrors arranged off-center.
From U.S. Pat. No. 5,581,605, an illumination system has been made known, in which a photon beam is split into a multiple number of secondary light sources by means of a plate with concave raster elements. In this way, a homogeneous or uniform illumination is achieved in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of a conventional reduction optics. A gridded mirror is precisely provided with equally curved elements in the illumination beam path. The contents of the above-mentioned patents are incorporated by reference.
The invention provides an illumination system for microlithography that fulfills the requirements for advanced lithography with wavelength less or equal to 193 nm.
The system illuminates a structured reticle arranged in the image plane of the illumination system, which will be imaged by a projection objective onto a light sensitive substrate. In stepper-type lithography systems the reticle is illuminated with a rectangular field, wherein a pregiven uniformity of the light intensity inside the field is required, for example better than xc2x15%. In scanner-type lithography systems the reticle is illuminated with a rectangular or arc-shaped field, wherein a pregiven uniformity of the scanning energy distribution inside the field is required, for example better than xc2x15%. The scanning energy is defined as the line integral over the light intensity in the scanning direction. The shape of the field is dependent on the type of projection objective. All reflective projection objectives typically have an arc-shaped field, which is given by a segment of an annulus. A further requirement is the illumination of the exit pupil of the illumination system, which is located at the entrance pupil of the projection objective. A nearly field-independent illumination of the exit pupil is required.
Typical light sources for wavelengths between 100 nm and 200 nm are excimer lasers, for example an ArF-Laser for 193 nm, an F2-Laser for 157 nm, an Ar2-Laser for 126nm and an NeF-Laser for 109 nm. For systems in this wavelength region refractive components of SiO2, CaF2, BaF2 or other crystallites are used. Since the transmission of the optical materials deteriorates with decreasing wavelength, the illumination systems are designed with a combination of refractive and reflective components. For wavelengths in the EUV wavelength region, between 10 nm and 20 nm, the projection exposure apparatus is designed as all-reflective. A typical EUV light source is a Laser-Produced-Plasma-source, a Pinch-Plasma-Source, a Wiggler-Source or an Undulator-Source.
The light of this primary light source is collected by a collector unit and directed to a first optical element, wherein the collector unit and the first optical element form a first optical component. The first optical element is organized as a plurality of first raster elements and transforms, together with the collector unit, the primary light source into a plurality of secondary light sources. Each first raster element corresponds to one secondary light source and focuses an incoming ray bundle, defined by all rays intersecting the first raster element, to the corresponding secondary light source. The secondary light sources are arranged in a pupil plane of the illumination system or nearby this plane. A field lens forming a second optical component is arranged between the pupil plane and the image plane of the illumination system to image the secondary light sources into an exit pupil of the illumination system, which corresponds to the entrance pupil of a following projection objective. The images of the secondary light sources in the exit pupil of the illumination system are therefore called tertiary light sources.
The first raster elements are imaged into the image plane, wherein their images are at least partially superimposed on a field that must be illuminated. Therefore, they are known as field raster elements or field honeycombs. If the light source is a point-like source, the secondary light sources are also point-like. In this case the imaging of each of the field raster elements can be explained visually with the principle of a xe2x80x9ccamera obscuraxe2x80x9d, with the small hole of the camera obscura at the position of each corresponding secondary light source, respectively.
To superimpose the images of the field raster elements in the image plane of the illumination system the incoming ray bundles are deflected by the field raster elements with first deflection angles, which are not equal for each of the field raster elements but at least different for two of the field raster elements. Therefore individual deflection angles for the field raster elements are designed.
For each field raster element a plane of incidence is defined by the incoming and deflected centroid ray selected from the incoming ray. bundle. Due to the individual deflection angles, at least two of the incidence planes are not parallel.
In advanced microlithography systems the light distribution in the entrance pupil of a projection objective must fulfill special requirements such as having an overall shape or uniformity. Since the secondary light sources are imaged into the exit pupil, their arrangement in the pupil plane of the illumination system determines the light distribution in the exit pupil. With the individual deflection angles of the field raster elements a predetermined arrangement of the secondary light sources can be achieved, independent of the directions of the incoming ray bundles.
For reflective field raster elements the deflection angles are generated by the tilt angles of the field raster elements. The tilt axes and the tilt angles are determined by the directions of the incoming ray bundles and the positions of the secondary light sources, to which the reflected ray bundles are directed.
For refractive field raster element the deflection angles are generated by lenslets, which have a prismatic optical power. The refractive field, raster elements can be lenslets with an optical power having a prismatic contribution or they can be a combination of a single prism and a lenslet. The prismatic optical power is determined by the directions of the incoming ray bundles and the positions of the corresponding secondary light sources.
Given the individual deflection angles of the first raster elements, the beam path to the plate with the raster elements can be either convergent or divergent. The slope values of the field raster elements at the centers of the field raster elements has then to be similar to the slope values of a surface with negative power to reduce the convergence of the beam path, or with positive power to increase the divergence of the beam path. Finally the field raster elements deflect the incoming ray bundles to the corresponding secondary light sources having predetermined positions depending on the illumination mode of the exit pupil.
The diameter of the beam path is preferably reduced after the collector unit to arrange filters or transmission windows with a small size. This is possible by imaging the light source with the collector unit to an intermediate image. The intermediate image is arranged between the collector unit and the plate with the field raster elements. After the intermediate image of the light source, the beam path diverges. An additional mirror to condense the diverging rays is not necessary due to the field raster elements having deflecting optical power
For contamination reasons there is a free working distance between the light source and the collector unit, which results in considerable diameters for the optical components of the collector unit and also for the light beam. Therefore the collector unit has positive optical power to generate a converging ray bundle to reduce the beam diameter and the size of the plate with field raster elements. The convergence of the light rays can be reduced with the field raster elements, if the deflection angles are designed to represent a negative optical power. For the centroid rays impinging on the centers of the field raster elements, the collector unit and the plate with the field raster elements form a telescope system. The collector unit has positive optical power to converge the centroid rays towards the optical axis, wherein the field raster elements reduce the converging angles of the centroid rays. With this telescope system the track length of the illumination system can be reduced.
Preferably, the field raster elements are tilted planar mirrors or prisms with planar surfaces, which are much easier to produce and to qualify than curved surfaces. This is possible, if the collector unit is designed to image the primary light source into the pupil plane of the illumination system, which would result in one secondary light source, if the field raster elements were omitted. The plurality of secondary light sources is generated by the plurality of field raster elements, which distribute the secondary light sources in the pupil plane according to their deflection angles. The positive optical power to focus the incoming ray bundles to the secondary light sources is completely provided by the collector unit. Therefore the optical distance between the image-side principal plane of the collector unit and the image plane of the collector unit is nearly given by the sum of the optical distance between the image-side principal plane of the collector unit and the plate with the field raster elements, and the optical distance between the plate with the field raster elements and the pupil plane of the illumination system. Due to the planar surfaces, the field raster elements do not influence the imaging of the primary light source into one secondary light source, except for the dividing of this one secondary light source into a plurality of secondary light sources due to the deflection angles. For point-like or spherical sources the collector unit has ellipsoidal mirrors or conical lenses with a first or second focus, wherein the primary light source is arranged in the first focus, and the secondary light source is arranged in the second focus of the collector unit.
Dependent on the focusing optical power of the collector unit, the field raster elements can have a positive or negative optical power. If the focusing power of the collector unit is too low and the primary light source is imaged behind the pupil plane, the field raster elements are preferably concave mirrors or lenslets comprising positive optical power to generate the secondary light sources in or nearby the pupil plane. If the focusing power of the collector unit is too strong and the primary light source is imaged in front of the pupil plane, the field raster elements are preferably convex mirrors or lenslets comprising negative optical power to generate the secondary light sources in or nearby the pupil plane.
The field raster elements are preferably arranged in a two-dimensional array on a plate without overlapping. For reflective field raster elements the plate can be a planar plate or a curved plate. To minimize the light losses between adjacent field raster elements they are arranged only with intermediate spaces between them, which are necessary for the mountings of the field raster elements. Preferably, the field raster elements are arranged in a plurality of rows having at least one field raster element and being arranged among one another. In the rows the field raster elements are put together at the smaller side of the field raster elements. At least two of these rows are displaced relative to one another in the direction of the rows. In one embodiment each row is displaced relative to the adjacent row by a fraction of a length of the field raster elements to achieve a regular distribution of the centers of the field raster elements. The fraction is dependent on the side aspect ratio and is preferably equal to the square root of the length of one field raster element. In another embodiment the rows are displaced in such a way that the field raster elements are illuminated almost completely.
Preferably, only these field raster elements are imaged into the image plane, which is completely illuminated. This can be realized with a masking unit in front of the plate with the field raster elements, or with an arrangement of the field raster elements wherein 90% of the field raster elements are completely illuminated.
It is advantageous to insert a second optical element with second raster elements in the light path after the first optical element with first raster elements, wherein one first raster element corresponds to one of the second raster elements. Therefore, the deflection angles of the first raster elements are designed to deflect the ray bundles impinging on the first raster elements to the corresponding second raster elements.
The second raster elements are preferably arranged at the secondary light sources and are designed to image together with the field lens the first raster elements or field raster elements into the image plane of the illumination system, wherein the images of the field raster elements are at least partially superimposed. The second raster elements are called pupil raster elements or pupil honeycombs. To avoid damaging the second raster elements due to the high intensity at the secondary light sources, the second raster elements are preferably arranged defocused of the secondary light sources, but in a range from 0 mm to 10% of the distance between the first and second raster elements.
For extended secondary light sources the pupil raster elements preferably have a positive optical power to image the corresponding field raster elements, which are arranged optically conjugated to the image plane. The pupil raster elements are concave mirrors or lenslets with positive optical power.
The pupil raster elements deflect incoming ray bundles impinging on the pupil raster elements with second deflection angles in such a way that the images of the field raster elements in the image plane are at least partially superimposed. This is the case if a ray intersecting the field raster element and the corresponding pupil raster element in their centers intersects the image plane in the center of the illuminated field or nearby the center. Each pair of a field raster element and a corresponding pupil raster element forms a light channel.
The second deflection angles are not equal for each pupil raster element. They are preferably individually adapted to the directions of the incoming ray bundles and the requirement to superimpose the images of the field raster elements at least partially in the image plane.
With the tilt axis and the tilt angle for a reflective pupil raster element or with the prismatic optical power for a refractive pupil raster element the second deflection angle can be individually adapted.
For point-like secondary light sources the pupil raster elements only have to deflect the incoming ray bundles without focusing the rays. Therefore the pupil raster elements are preferably designed as tilted planar mirrors or prisms.
If both, the field raster elements and the pupil raster elements deflect incoming ray bundles in predetermined directions, the two-dimensional arrangement of the field raster elements can be made different from the two-dimensional arrangement of the pupil raster elements. Wherein the arrangement of the field raster elements is adapted to the illuminated area on the plate with the field raster elements, the arrangement of the pupil raster elements is determined by the kind of illumination mode required in the exit pupil of the illumination system. So the images of the secondary light sources can be arranged in a circle, but also in an annulus to get an annular illumination mode or in four decentered segments to get a Quadrupol illumination mode. The aperture in the image plane of the illumination system is approximately defined by the quotient of the half diameter of the exit pupil of the illumination system and the distance between the exit pupil and the image plane of the illumination system. Typical apertures in the image plane of the illumination system are in the range of 0.02 and 0.1. By deflecting the incoming ray bundles with the field and pupil raster. elements a continuous light path can be achieved. It is also possible to assign each field raster element to any of the pupil raster elements. Therefore the light channels can be mixed to minimize the deflection angles or to redistribute the intensity distribution between the plate with the field raster elements and the plate with the pupil raster elements.
Imaging errors such as distortion introduced by the field lens can be compensated for with the pupil raster elements being arranged at or nearby the secondary light sources. Therefore the distances between the pupil raster elements are preferably irregular. The distortion due to tilted field mirrors for example is compensated for by increasing the distances between the pupil raster elements in a direction perpendicular to the tilt axis of the field mirrors. Also, the pupil raster elements are arranged on curved lines to compensate for the distortion due to a field mirror, which transforms the rectangular image field to a segment of an annulus by conical reflection. By tilting the field raster elements the secondary light sources can be positioned at or nearby the distorted grid of the corresponding pupil raster elements.
For reflective field and pupil raster elements the beam path has to be folded at the plate with the field raster elements and at the plate with the pupil raster elements to avoid vignetting. Typically, the folding axes of both plates are parallel. Another requirement for the design of the illumination system is to minimize the incidence angles on the reflective field and pupil raster elements. Therefore the folding angles have to be as small as possible. This can be achieved if the extent of the plate with the field raster elements is approximately equal to the extent of the plate with the pupil raster elements in a direction perpendicular to the direction of the folding axes, or if it differs less than xc2x110%.
Since the secondary light sources are imaged into the exit pupil of the illumination system, their arrangement determines the illumination mode of the pupil illumination. Typically the overall shape of the illumination in the exit pupil is circular and the diameter of the illuminated region is in the order of 60%-80% of the diameter of the entrance pupil of the projection objective. The diameters of the exit pupil of the illumination system and the entrance pupil of the projection objective are in another embodiment preferably equal. In such a system the illumination mode can be changed in a wide range by inserting masking blades at the plane with the secondary light sources to get a conventional, Dipol or Quadrupol illumination of the exit pupil.
All-reflective projection objectives used in the EUV wavelength region have typically an object field being a segment of an annulus. Therefore the field in the image plane of the illumination system in which the images of the field raster elements are at least partially superimposed has preferably the same shape. The shape of the illuminated field can be generated by the optical design of the components or by masking blades which have to be added nearby the image plane or in a plane conjugated to the image plane.
The field raster elements are preferably rectangular. Rectangular field raster elements have the advantage that they can be arranged in rows being displaced against each other. Depending on the field to be illuminated they have a side aspect ratio in the range of 5:1 and 20:1. The length of the rectangular field raster elements is typically between 15 mm and 50 mm, the width is between 1 mm and 4 mm.
To illuminate an arc-shaped field in the image plane with rectangular field raster elements the field lens preferably comprises a first field mirror for transforming the rectangular images of the rectangular field raster elements to arc-shaped images. The arc length is typically in the range of 80 mm to 105 mm, the radial width in the range of 5 mm to 9 mm. The transformation of the rectangular images of the rectangular field raster elements can be done by conical reflection with the first field mirror being a grazing incidence mirror with negative optical power. In other words, the imaging of the field raster elements is distorted to get the arc-shaped images, wherein the radius of the arc is determined by the shape of the object field of the projection objective. The first field mirror is preferably arranged in front of the image plane of the illumination system, wherein there should be a free working distance. For a configuration with a reflective reticle the free working distance has to be adapted to the fact that the rays traveling from the reticle to the projection objective are not vignetted by the first field mirror.
The surface of the first field mirror is preferably an off-axis segment of a rotational symmetric reflective surface, which can be designed aspherical or spherical. The axis of symmetry of the supporting surface goes through the vertex of the surface. Therefore a segment around the vertex is called on-axis, wherein each segment of the surfaces which does not include the vertex is called off-axis. The supporting surface can be manufactured more easily due to the rotational symmetry.
After producing the supporting surface the segment can be cut out with well-known techniques.
The surface of the first field mirror can also be designed as an on-axis segment of a toroidal reflective surface. Therefore the surface has to be processed locally, but has the advantage that the surrounding shape can be produced before surface treatment.
The incidence angles of the incoming rays with respect to the surface normals at the points of incidence of the incoming rays on the first field mirror are preferably greater than 70xc2x0, which results in a reflectivity of the first field mirror of more than 80%.
The field lens comprises preferably a second field mirror with positive optical power. The first and second field mirror together image the secondary light sources or the pupil plane respectively into the exit pupil of the illumination system, which is defined by the entrance pupil of the projection objective. The second field mirror is arranged between the plane with the secondary light sources and the first field mirror.
The second field mirror is preferably an off-axis segment of a rotational symmetric reflective surface, which can be designed aspherical or spherical, or an on-axis segment of a toroidal reflective surface.
The incidence angles of the incoming rays with respect to the surface normals at the points of incidence of the incoming rays on the second field mirror are preferably lower than 25xc2x0. Since the mirrors have to be coated with multilayers for the EUV wavelength region, the divergence and the incidence angles of the incoming rays are preferably as low as possible to increase the reflectivity, which should be better than 65%. With the second field mirror being arranged as a normal incidence mirror the beam path is folded and the illumination system can be made more compact.
To reduce the length of the illumination system the field lens comprises preferably a third field mirror. The third field mirror is preferably arranged between the plane with the secondary light. sources and the second field mirror.
The third field mirror has preferably negative optical power and forms together with the second and first field mirror an optical telescope system having a object plane at the secondary light sources and an image plane at the exit pupil of the illumination system to image the secondary light sources into the exit pupil. The pupil plane of the telescope system is arranged at the image plane of the illumination system. Therefore the ray bundles coming from the secondary light sources are superimposed in the pupil plane of the telescope system or in the image plane of the illumination system accordingly. The first field mirror has mainly the function of forming the arc-shaped field, wherein the telescope system is mainly determined by the negative third field mirror and the positive second field mirror.
In another embodiment the third field mirror has preferably positive optical power to generate images of the secondary light sources in a plane between the third and second field mirror, forming tertiary light sources. The tertiary light sources are imaged with the second field mirror and the first field mirror into the exit pupil of the illumination system. The images of the tertiary light sources in the exit pupil of the illumination system are called in this case quaternary light sources.
Since the plane with the tertiary light sources is arranged conjugated to the exit pupil, this plane can be used to arrange masking blades to change the illumination mode or to add transmission filters. This position in the beam path has the advantage to be freely accessible.
The third field mirror is similar to the second field mirror preferably an off-axis segment of a rotational symmetric reflective surface, which can be designed aspherical or spherical, or an on-axis segment of a toroidal reflective surface.
The incidence angles of the incoming rays with respect to the surface normals at the points of incidence of the incoming rays on the third field mirror are preferably lower than 25xc2x0. With the third field mirror being arranged as a normal incidence mirror the beam path can be folded again to reduce the overall size of the illumination system.
To avoid vignetting of the beam path the first, second and third field mirrors are preferably arranged in a non-centered system. There is no axis of symmetry for the mirrors. An optical axis can be defined as a connecting line between the centers of the used areas on the field mirrors, wherein the optical axis is bent at the field mirrors depending on the tilt angles of the field mirrors.
With the tilt angles of the reflective components of the illumination system the beam paths between the components can be bent. Therefore the orientation of the beam cone emitted by the source and the orientation of the image plane system can be arranged according to the requirements of the overall system. A preferable configuration has a source emitting a beam cone in one direction and an image plane having a surface normal which is oriented almost perpendicular to this direction. In one embodiment the source emits horizontally and the image plane has a vertical surface normal. Some light sources like undulator or wiggler sources emit only in the horizontal plane. On the other hand the reticle should be arranged horizontally for gravity reasons. The beam path therefore has to be bent between the light source and the image plane about almost 90xc2x0. Since mirrors with incidence angles between 30xc2x0 and 60xc2x0 lead to polarization effects and therefore to light losses, the beam bending has to be done only with grazing incidence or normal incidence mirrors. For efficiency reasons the number of mirrors has to be as small as possible.
A very compact configuration of the illumination system can be designed, if the beam path from the plate with the pupil raster elements to the field lens is crossing the beam path from the collector unit to the plate with field raster elements. This is only possible, if the field raster elements and the pupil raster elements are reflective ones and are arranged on plates being tilted to achieve the crossing of the two beam paths. The crossing of the beam paths has the advantage that the beam path after the plate with the pupil raster elements has an angle in the range of 35xc2x0 to 55xc2x0 with respect to the beam path in front of the plate with the field raster elements. This was achieved with only two normal incidence reflections.
By definition all rays intersecting the field in the image plane have to go through the exit pupil of the illumination system. The position of the field and the position of the exit pupil are defined by the object field and the entrance pupil of the projection objective. For some projection objectives being centered systems the object field is arranged off-axis of an optical axis, wherein the entrance pupil is arranged on-axis in a finite distance to the object plane. For these projection objectives an angle between a straight line from the center of the object field to the center of the entrance pupil and the surface normal of the object plane can be defined. This angle is in the range of 3xc2x0 to 10xc2x0 for EUV projection objectives. Therefore the components of the illumination system have to be configured and arranged in such a way that all rays intersecting the object field of the projection objective are going through the entrance pupil of the projection objective being decentered to the object field. For projection exposure apparatus with a reflective reticle all rays intersecting the reticle needs to have incidence angles greater than 0xc2x0 to avoid vignetting of the reflected rays at components of the illumination system.
In the EUV wavelength region all components are reflective components, which are arranged preferably in such a way, that all incidence angles on the components are lower than 25xc2x0 or greater than 65xc2x0. Therefore polarization effects arising for incidence angles around an angle of 45xc2x0 are minimized. Since grazing incidence mirrors have a reflectivity greater than 80%, they are preferable in the optical design in comparison to normal incidence mirrors with a reflectivity greater than 65%.
The illumination system is typically arranged in a mechanical box. By folding the beam path with mirrors the overall size of the box can be reduced. This box preferably does not interfere with the image plane, in which the reticle and the reticle supporting system are arranged. Therefore it is advantageous to arrange and tilt the reflective components in such a way that all components are completely arranged on one side of the reticle. This can be achieved if the field lens comprises only an even number of normal incidence mirrors.
The illumination system as described before can be used preferably in a projection exposure apparatus comprising the illumination system, a reticle arranged in the image plane of the illumination system and a projection objective to image the reticle onto a wafer arranged in the image plane of the projection objective. Both, reticle and wafer are arranged on a support unit, which allows the exchange or scan of the reticle or wafer.
The projection objective can be a catadioptric lens, as known from U.S. Pat. No. 5,402,267 for wavelengths in the range between 100 nm and 200 nm. These systems have typically a transmission reticle.
For the EUV wavelength range the projection objectives are preferably all-reflective systems with four to eight mirrors as known for example from U.S. Ser. No. 09/503640 showing a six mirror projection lens. These systems have typically a reflective reticle.
For systems with a reflective reticle the illumination beam path between the light source and the reticle and the projection beam path between the reticle and the wafer preferably interfere only nearby the reticle, where the incoming and reflected rays for adjacent object points are traveling in the same region. If there are no further crossing of the illumination and projection beam path it is possible to separate the illumination system and the projection objective except for the reticle region.
The projection objective has preferably a projection beam path between said reticle and the first imaging element which is convergent toward the optical axis of the projection objective. Especially for a projection exposure apparatus with a reflective reticle the separation of the illumination system and the projection objective is easier to achieve.